One Core Hotter and slower than others

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I overclocked my Intel Core i5 3570K to 4.2GHz at 1.1V. It ran stably overnight on Prime95, but, while I was monitoring the core temps, they stuck within 45-55C with core #3 was always 3 or 4 degrees higher than all the other cores. Then after 11:50 Hours I stopped the workers and saw that all cores completed around 810, except core 3 which did 707 tests.
Do I have a defective CPU? Background Processes? Or should I higher (or lower) slightly the voltage?
(HowTos)

Though it would be very slightly slower in multithreaded loads, I think I would prefer an i7 with one core disabled to having hyperthreading disabled. 3 strong cores and the ability to run 6 threads sounds about right to me. Intel could probably salvage more dud i7's this way too, resulting in either higher margins or lower prices for us.